1. Field of the Invention
The present invention relates to an image forming apparatus for correcting a trapping width for curving of a scanning line that occurs when an image is formed and for performing a trapping process, and to a method for performing a trapping process with the image forming apparatus.
2. Description of the Related Art
Recently, in order to increase the speed of image formation in electrophotographic color image forming apparatuses (laser printers), a tandem electrophotographic method has been employed in most cases. In this method, developing devices and photosensitive drums are arranged, the number of developing devices and the number of photosensitive drums being the same as the number of color materials (toners). Images having different colors are sequentially transferred onto a conveying belt or a recording sheet, thereby forming a color image. In such a tandem color image forming apparatus, because images having different colors are formed by image forming units that correspond to the colors, a shift between color plates may occur in terms of the images having different colors. Because of such a shift between color plates, a white region may appear between an image portion having a certain color and an image portion having a different color, or a portion in which a specific color is more conspicuous because of an overlap between an image portion having a certain color and an image portion having a different color may appear.
Furthermore, as factors responsible for such a shift between color plates, nonuniformity among lenses, each of which is provided in a deflection scanner that performs deflection scanning with laser light to form a latent image, a shift of a position at which the lens is mounted, a shift of a position at which the deflection scanner is installed in a body of the image forming apparatus, and so forth can be considered. In the case of such an occurrence, inclining or curving of a scanning line of laser light occurs. Additionally, because the degree of inclination or curving of a scanning line differs for each color, a shift between plates appears as a result.
Various methods for dealing with such a shift between color plates have been proposed.
In Japanese Patent Laid-Open No, 2002-116394, the degree of curving of a scanning line in a deflection scanner is measured using an optical sensor in a process of assembling the deflection scanner. The curvature of the scanning line is adjusted by rotating a lens in a mechanical manner in accordance with a result of measurement, and then, the lens or the like is fixed using an adhesive.
Furthermore, in Japanese Patent Laid-Open No, 2003-241131, the degree of inclination of a scanning line is measured using an optical sensor when a deflection scanner is installed in a body of a color image forming apparatus. The inclination of the scanning line is adjusted by inclining the deflection scanner in a mechanical manner in accordance with the degree of the inclination. After the inclination of the scanning line is adjusted in this manner, the deflection scanner is installed in the color image forming apparatus, and fixed.
In addition to the method for dealing with such a shift between color plates, which requires mechanical adjustment or an adjustment process in assembly as disclosed in Japanese Patent Laid-Open No, 2002-116394 or No, 2003-241131, a trapping technique has been known. The trapping technique involves setting of, at a boundary between two adjacent objects having different colors, a portion in which the objects slightly overlap each other (hereinafter, referred to as “trapping”).
Moreover, in order to optimally perform trapping, it is necessary that a trapping width be set to be large at a position at which a large white region appears, and that the trapping width be set to be small at a position at which an overlap between color images appears.
As a method for performing an appropriate trapping process, Japanese Patent Laid-Open No, 2007-221226 discloses the following method: a sheet is divided into a plurality of division regions in advance; an amount of a shift between plates that appears for each of the division regions is obtained; trapping characteristic data is obtained in accordance with the amount of a shift between plates for each of the division regions; and a trapping width that is determined from the obtained trapping characteristic data is applied for trapping.
The degree of inclining or curving of a scanning line of laser light differs for each color. When a scanning line of laser light that is used to generate a plate having a color is curved, depending on curving of the scanning line of laser light for a color for which trapping is to be performed, different trapping widths, e.g., a trapping width for a case in which trapping is performed with upward growth and a trapping width for a case in which trapping is performed with downward growth, may be necessary. Herein, trapping with upward growth indicates increasing of a region in which toner having a certain color is fixed so that an adjacent region in which toner having a different color is fixed overlaps the region in which toner having a certain color is fixed in a sub-scanning direction. Furthermore, increasing of a region in which toner having a certain color is fixed so that an adjacent region in which toner having a different color is fixed overlaps the region in which toner having a certain color is fixed in a direction that is opposite to the sub-scanning direction is called trapping for downward growth. For example, as shown in FIG. 6, it is supposed that a region having a certain color and a region having a different color exist, which are adjacent to each other, and that trapping is performed for the region having a different color. With respect to a curved scanning line of laser light for the certain color, a scanning line of laser light for the different color is curved in the sub scanning direction (the downward direction) as the scanning line of laser light for the different color extends in a main scanning direction. In this case, as shown in FIG. 7, the necessary trapping width for upward growth is decreased in the main scanning direction (the right direction), and appearance of a white spot cannot be prevented with the trapping width for upward growth. The necessary trapping width for downward growth is increased, so that the trapping width for downward growth is larger than necessary. Note that, in this case, an image portion having a dark color shown in part (A) of FIG. 7 is formed using laser light for a certain color that is a criterion. Image portions that are shown in part (A) of FIG. 7 and that sandwich the image portion having a dark color are formed using laser light for a different color.
In the method disclosed in Japanese Patent Laid-Open No, 2007-221226, in accordance with information concerning a shift between plates, which is obtained for each region, trapping is performed with a fixed trapping width at a color boundary that is found in the region. Accordingly, different trapping widths, e.g., a trapping width for upward growth and a trapping width for downward growth, cannot be set. Thus, the trapping width that is larger than necessary is set, and this leads to reduction in trapping quality.